Why is it not recommended to stabilize cells with RNAprotect Tissue Reagent?
FAQ ID -941

RNAprotect Tissue Reagent is recommended for stabilization and protection of RNA in animal tissues. It is not recommended for cultured cells because it may be difficult to pellet and remove them from the reagent prior to RNA isolation (see FAQ 364). 

Instead, we recommend to use the RNAprotect Cell Reagent.  This reagent is for immediate RNA stabilization in sorted or cultured cells with no need to remove medium